We propose the synthesis of a new fluorophore for intracellular glutathione determination. The reaction product of the fluorophore with thiols will have longer wavelength absorption and emission and higher quantum yield than the best cytosolic thiol fluorophore currently in use for cell sorting by flow cytometry. The dye will have the advantage of being cell membrane permeable owing to esterified phenol and carboxyl groups which will subsequently be de-esterified by cytosolic esterases, thus trapping the indicator within the cell. Therefore, cell loading should occur at much lower extracellular dye concentrations than currently possible. We also propose to study the absorption and emission properties in both bound and unbound states with glutathione and thiol- containing proteins.